Linear accelerator slow wave structure



March 8, 1966 N. J. NORRIS LINEAR ACCELERATOR SLOW WAVE STRUCTURE Filed Nov. 2, 1961 United States Patent 3,239,712 LINEAR ACCELERATOR SLOW WAVE STRUCTURE Neil J. Norris, Walnut Creek, Califi, assignor, by mesne assignments, to High Voltage Engineering Corporation,

Burlington, Mass., a corporation of Massachusetts Filed Nov. 2, 1961, Ser. No. 149,760 2 Claims. (Cl. 315--3.5)

This invention relates generally to linear accelerator slow wave structures and in particular to improvements thereon for the purpose of providing more efiicient and more flexible beam focusing capabilities.

A linear accelerator is a device which projects a beam of charged particles in close proximity to an electromagnetic wave traveling in a waveguide or other suitable transmission structure in such a manner that interaction between the beam and the electromagnetic wave accelerates the beam particles. In such a device, effective interaction between electromagnetic wave and beam can be obtained only if they progress down the transmission structure at substantially the same speed. To accomplish this, the transmission structure, usually a circular waveguide, is provided with a slow wave structure comprising a series of axially aligned, apertured discs periodically located along its length, whereby the faster moving electromagnetic wave is retarded to approximate the speed of the beam. The beam which passes freely through the disc apertures is, however, subject to space charge forces which tend to diffuse it. Consequently, many focusing systems have been devised to confine and shape such a beam. The focusing devices currently in use are inherently cumbersome, inflexible and power consuming. There therefore exists a need for more efficient and more flexible focusing systems in the linear accelerator art, and it is to this end that my invention is directed.

Accordingly, it is a principal object of my invention to better utilize the slow wave structure of a linear accelerator to provide a low reluctance path for the more effective application of the beam focusing flux, thereby reducing the size and power requirements of the focusing means.

It is a further object of my invention to provide, in a linear accelerator, a novel slow wave structure having a plurality of discrete low reluctance sectors as integral parts of the periodical loading discs, said low reluctance sectors being adapted to direct and concentrate the beam focusing fiux so as to effect a variety of beam configurations.

It is a still further object of my invention to provide a novel slow wave structure adapted to give greater flexibility and efficiency to linear accelerator focusing systems, said novel slow wave structure being compatible with and adaptable to essentially all existing focusing systems.

These, together with other objectsand features, will become more apparent as my invention is set forth more fully in conjunction with the accompanying drawings, wherein:

FIGURE 1 is a longitudinal section of a linear accelerator embodying the principles of my invention;

FIGURE 2 illustrates a periodic loading disc as comprehended by one embodiment of my invention; and,

FIGURE 3 is a section through said periodic loading disc taken at 33.

Referring now to FIGURE 1, there is illustrated a linear accelerator of the type comprehended by my invention, which includes evacuated tubular waveguide 4, a plurality of transverse longitudinally spaced discs 6, electron beam source 5, electromagnetic wave source 8, output window 12, and beam focusing means 9. Tubular waveguide 4 and discs 6 are made of copper or some other material having good electrical conductivity. Discs 6 have axially aligned apertures 14 therein for the purpose of permit- 3,239,712 Patented Mar. 8, 1956 "ice ting the free passage of electron beam 7 therethrough. This disc arrangement, which retards the electromagnetic wave supplied from electromagnetic wave source 8, in combination with tubular waveguide 4 comprises the slow wave structure. In operation, electron beam 7 is projected through the waveguide along its longitudinal axis. Electromagnetic energy is supplied from source 8, commonly a magnetron, through coupling loop 10. The electromagnetic wave, as it passes along tube 4 is retarded by discs 6, such that its phase velocity is substantially equal to the velocity of the particles in beam 7. The subsequent interaction between the traveling electromagnetic wave and the beam thus accelerates some of the beam particles to high energy values. An output window 12 of aluminum or some similar material through which the accelerated particles may pass, is provided such that the beam may be directed to a target 13.

My invention is particularly concerned with improving the efficiency of the focusing means, which means, in the present illustrative embodiment comprises a plurality of solenoids 9 arranged to provide a focusing flux along the longitudinal axis of envelope 4. Because of the relatively long distance over which electron beam 7 must be projected and because of large space charge forces existing in such a high density electron beam, the focusing means, if it is to be of sufiicient strength to confine the beam, must, of necessity, be large and bulky in com parison with the linear accelerator itself. The present state of the art reveals that many focusing schemes have been developed to alleviate this situation. Replacing the solenoid with permanent magnets, providing an axially symmetric periodic magnetic field, providing a quadrupolar periodic magnetic field, and application of the strong focusing principle, are typical of solutions that have to some extent reduced the size and power requirements of linear accelerator focusing systems. My invention therefore comprehends a novel slow wave structure which in combination with essentially any conventional focusing system provides a refinement thereof adapted to further reduce weight and power requirements. My invention further provides a wider variety of field configurations than has heretofore been possible.

Since the power required of the focusing means is a function of the distance of the solenoids or permanent magnets from the axis of the tube, I propose to effectively reduce the distance in an electrical sense by providing a plurality of low reluctance magnetic paths therebetween. In the present illustrative embodiment of FIGURE 1, this is achieved by fabricating loading discs 6 of a highly permeable metal such as steel or permalloy instead of the conventional copper. The loading discs thus fabricated are then plated with copper or other electrically conductive material in order to minimize radiofrequency losses. The effect of the magnetic material in the disc is to reduce the reluctance of the magnetic circuit and improve the efficiency of the focusing fields. By suitably placing the electrical coils or permanent magnets on the outside of tubular waveguide 4, the field on the axis thereof may be controlled by the use of the magnetic material in the discs. Such an arrangement also allows an alternating gradient field to be produced on the axis of tubular waveguide 4, if desired. This type of field has several advantages in producing focusing effects and small beam diameters that are not possible with solenoidal focusing. The alternating field produced in this way differs from the field produced with alternating solenoidal fields alone in that most of the radial field may be confined to the magnetic material. In this way, an alternating field may be produced that has a small axial distance of action, that is, the spacing between two adjacent discs as indicated by dimension a of FIGURE 1. Since said spacing between successive discs is comparable to the diameter of the aperture, dimension b, good magnetic properties are obtainable in the space occupied by the beam.

Another embodiment of my invention is illustrated in FIGURES 2 and 3. There is illustrated thereby loading disc 16 in relationship to evacuated tubular waveguide 4 and focusing magnets 1'7. In the instant embodiment, loading disc 16 is fabricated of copper having segments 18 of magnetic material cast therein. Said segments 18 may be shaped and oriented to provide the four quarter-section low reluctance paths for quadrupole focusing as illustrated by FIGURE 2, or they may take any other form or position adapted to accomplish a particular desired field configuration. This arrangement of the magnetic material within the copper disc permits the distribution of the flux field with angular as well as with axial position, thereby providing for a wide variety of field configurations. It is therefore readily apparent that by selective application of either electro magnets or permanent magnets external to the waveguide, effective alternate gradient and quadrupole focusing may be achieved through the use of a slow Wave structure consisting of a plurality of such loading discs.

Since many changes could be made in the above construction and many apparently widely dilferent embodiments of this invention could be made Without departing from the scope thereof, it is intended that all matter contained in the above description or in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a linear accelerator having an evacuated envelope,

means for coupling electromagnetic energy into said envelope, means for projecting a stream of charged particles therethrough, means for propagating an electromagnetic Wave in coupled relationship With said stream, a slow wave structure comprising a succession of annular members disposed within said envelope at periodic intervals in transverse relationship to said stream of charged particles, said annular members being fabricated of copper and each having a plurality of separated permeable metal segments cast therein, and means for providing a magnetic flux in said segments for focusing said stream of charged particles, said segments having geometric configurations adapted to define and orient said magnetic fiux, whereby beam focusing is enhanced.

2. Apparatus in accordance with claim 1 wherein said permeable metal segments are shaped and oriented to effect selective concentration of a magnetic focusing flux.

References Cited by the Examiner UNITED STATES PATENTS 5/1956 Robertson et al 328-233 9/1949 Anderson 3l5-39.71

OTHER REFERENCES ELI LIEBERMAN, Acting Primary Examiner. GEORGE N. WESTBY, Examiner. 

1. IN A LINEAR ACCELERATOR HAVING AN EVACUATED ENVELOPE, MEANS FOR COUPLING ELECTROMAGNETIC ENERGY INTO SAID ENVELOPE, MEANS FOR PROJECTING A STREAM OF CHARGED PARTICLES THERETHROUGH, MEANS FOR PROPAGATING AN ELECTROMAGNETIC WAVE IN COUPLED RELATIONSHIP WITH SAID STREAM, A SLOW WAVE STRUCTURE COMPRISING A SUCCESSION OF ANNULAR MEMBERS DISPOSED WITHIN SAID ENVELOPE AT PERIODIC INTERVALS IN TRANSVERSE RELATIONSHIP TO SAID STREAM OF CHARGED PARTICLES, SAID ANNULAR MEMBERS BEING FABRICATED OF COPPER AND EACH HAVING A PLURALITY OF SEPARATED PERMEABLE METAL SEGMENTS CAST THEREIN, AND MEANS FOR PROVIDING A MAGNETIC FLUX IN SAID SEGMENTS FOR FOCUSING SAID STREAM OF CHARGED PARTICLES, SAID SEGMENTS HAVING GEOMETRIC CONFIGURATIONS ADAPTED TO DEFINE AND ORIENT SAID MAGNETIC FLUX, WHEREBY BEAM FOCUSING IS ENHANCED. 